1. Field of the Invention
The present invention relates to a method for producing an electrophotographic toner, an electrophotographic toner, a full-color image forming method, and a full-color image forming apparatus.
2. Description of the Related Art
In recent years, in the field of an image forming technology utilizing electrophotography, there is an ever-increasing competition in the development of an apparatus for color image formation that can realize high-speed image formation and, at the same time, can yield a color image having high image quality (technology for forming high grade color image). For this reason, in order to form a full color image at a high speed, the so-called tandem system has become extensively adopted in methods for image formation. In the tandem system, a plurality of electrophotographic photoconductors (otherwise referred to as photoconductor or photoconductors, simply) are tandemly arranged. Images for respective color components are formed in respective electrophotographic photoconductors. The formed images are superimposed on top of each other on an intermediate transfer medium, and the superimposed images are transferred at a time on a recording medium (for example, Japanese Patent Application Laid-Open (JP-A) Nos. 07-209952 and 2000-075551).
The use of the intermediate transfer medium is effective in preventing the transfer of smear directly onto a recording medium such as paper when smear has occurred on the electrophotographic photoconductors during development. Since, however, in the system using the intermediate transfer medium, two transfer steps, that is, a step of transfer from the electrophotographic photoconductor to the intermediate transfer medium (primary transfer) and a step of transfer from the intermediate transfer medium to a recording medium to give a final image (secondary transfer), are performed, the transfer efficiency is lowered.
On the other hand, in addition to the above problem, there is a demand for the formation of a high-quality full color image. To meet this demand, a developer has been designed for improving an image quality. In order to cope with the demand for the improved image quality, particularly in a full color image, there is an increasing tendency toward the production of a toner having a smaller particle diameter, and studies have been made on faithful reproduction of a latent image to be formed on a photoconductor. Regarding the reduction in particle diameter, a process for producing a toner by a polymerization process has been proposed as a method that can regulate the toner so as to have a desired shape and surface structure (for example, Japanese Patent No. (JP-B) 3640918, and Japanese Patent Application Laid-Open (JP-A) No. 06-250439). In the toner produced by the polymerization process, in addition to the control of the diameter of toner particles, the shape of toner particles can also be controlled. A combination of this technique with a particle size reduction can improve the reproducibility of dots and narrow lines, and can reduce pile height (image layer thickness), whereby an improvement in image quality can be expected.
When a toner having a small particle diameter is used, however, non-electrostatic adhesion between the toner particle and the electrophotographic photoconductor or between the toner particle and the intermediate transfer medium is increased. Accordingly, the transfer efficiency is likely to be further lowered. This leads to such an unfavorable phenomenon that, when the toner having a small particle diameter is used in a high-speed full-color image forming apparatus, the transfer efficiency, particularly in the secondary transfer is significantly lowered. The reason for this is that the degree of difficulty of transfer is increased because, due to the reduction in particle diameter of the toner, the non-electrostatic adhesion to the intermediate transfer medium per toner particle is increased, a plurality of color toners are present in a superimposed state in the secondary transfer, and, due to an increase in speed, the period of time, for which the toner particle undergoes a transfer electric field in a nip portion in the secondary transfer, is decreased.
Further increasing the transfer electric field in the secondary transfer is considered effective in overcoming the above problem. However, when the transfer electric field is excessively increased, the transfer efficiency disadvantageously decreases. Accordingly, there is a limitation on this technique.
Prolonging the period of time for which the toner particle undergoes the transfer electric field by increasing the width of the nip portion in the secondary transfer is also considered. In a contact-type voltage application system using a bias roller and the like, in order to increase the nip width, only any one of a method in which the contact pressure of the bias roller is increased, and a method in which the roller diameter of the bias roller is increased, can be adopted. Increasing the contact pressure has a limitation from the viewpoints of image quality, and increasing the roller diameter has a limitation from the viewpoint of a reduction in size of the apparatus. In a non-contact-type voltage application system using a charger or the like, the nip width in the secondary transfer should be increased, for example, by increasing the number of chargers. Accordingly, this also has a limitation. For the above reason, it can be said that, particularly in high-speed machines, it is practically impossible to increase the nip width so as to obtain transfer efficiency higher than that in the present stage.
On the other hand, a method has been proposed in which the type and addition amount of additives are regulated (particularly, additives having a large particle diameter is added) as a method that reduces the non-electrostatic adhesion between a toner particle and an electrophotographic photoconductor or between the toner particle and an intermediate transfer medium (for example, JP-A No. 2001-066820 and JP-B No. 3692829). According to this method, by virtue of the non-electrostatic adhesion reduction effect, the toner particle can realize improvement in transfer efficiency. Further, in this method, additional effects such as stability of development and improvement in cleaning effect can be attained.